Shadow work would best be done outside, but it isn't necessary. You can do this work inside just as well.

The object here is to draw out the left side during wakefulness. Get a comfortable spot with the lights off, or only one soft one, on somewhere that will create shadows, deep shadows that run all the way to blackness in the room.

Mirror scrying is an ancient Celtic tool, but it's been used in all shamanic traditions at one time or another. The Celts just did more crazy things with it. Originally, its purpose was related to the military, as in looking into a pool of water to see an enemy's actions and/or shape shifting to a strategic location.